This invention relates generally to geophones, and more particularly to suspension springs for geophones or seismometers.
Geophones are devices which sense motion by suspending an inertial mass structure from a rigid, fixed reference supporting structure. Typically, the mass is a coil suspended by springs in a magnetic field, one spring being attached at each end of the coil. The springs position the coil within the magnetic field and form a suspension system having a pre-determined resonant frequency.
In present day geophones, spider springs are used having an inner ring and an outer ring which are connected by a plurality of spokes or legs. Generally three such legs are used, and the three-leg arrangement is generally considered the most stable.
The geophone is intended to sense movement from only one direction. Therefore, it is desirable to eliminate or minimize the effects of any motion from a direction not parallel to the axis of movement of the suspended coil within the geophone. The typical spring suspension system, however, produces a component of vibration along the axis of movement of the suspended coil when the geophone is moved laterally. This produces an undesired or false signal which is commonly referred to in the art as "spurious resonance".
Inasmuch as the geometry and the mass (coil) for a given model of geophone are constant within manufacturing limits, the spurious resonance is also constant. The frequency of the spurious resonance can therefore be raised or lowered by changing the geometry of the suspension springs. This characteristic has been used to segregate or eliminate false signals which are generated by lateral movement of the geophone. Typically, the problem is handled by raising the frequency of the spurious resonance to a level well above the frequency spectrum of interest for the geophone. The present state of the art normally requires a ratio of about 30 to 1 of the spurious resonance to the natural frequency of the geophone.
A common method of achieving the desired ratio of spurious resonance to natural frequency is to shorten the spring legs, thereby raising the frequency of the spurious resonance. This method, while effective, has undesirable side effects.
The legs of known prior art geophone springs have a rectangular cross-section and are curved along their lengths between the junctures with the inner and outer rings of the spring. The spring is "pre-formed", that is, the inner ring is offset or displaced relative to the outer ring, such that when the mass is suspended between two such springs, the inner ring, legs, and outer ring of each spring lie in the same plane. Such pre-forming causes stresses to occur within the spring legs. If the stresses are not uniformly distributed throughout the spring legs, portions of the legs will reach the yield point of the material before other portions with the consequence that the spring legs will become distorted. Such distortions can cause the spring to function in a non-linear manner, thereby producing a distorted signal from the geophone.
The signal distortion caused by spring non-linearity is increased when the legs are relatively short and the pre-forming large, precisely the structure found in low frequency geophones with high spurious resonance. The present invention helps eliminate such signal distortion and spring non-linearity.